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What is the part# of a blank cover control plate for the NEMA 1 boxes?

It depends on when the starter was made.  Prior to 2015, the NEMA 1,  Box 1 design used the C400 type cover control kits, which were long rectangular kits.  The blank cover to replace one of those kits would be part# C400GK0.  If it is the new style NEMA 1 box that used the round, M22 devices on it, THAT type if you wanted to remove a switch, light or button, you would just use a 22mm hole plug to plug up that hole.  Part#  M22S-B for black hole plug.